Handheld devices have been developed for a wide variety of applications, which include, for example, cell phones, personal digital assistants (PDAs), handheld computer terminals, scanners, and RFID readers/writers. Such handheld devices generally have a computer circuit on the main logic board (MLB), one or multiple radio systems for sharing data with other computers over the internet, cellular or proprietary networks, and interfaces for communicating with external devices, together with other components having user interfaces (e.g., displays, keyboards).
Since there is a demand to reduce the size of handheld devices, components tend to be placed closely together in the package of the handheld device. This increases the chances of electromagnetic energy emitted from one circuit getting into another and possibly causing performance degradation. This electromagnetic interference (EMI) includes radio frequency interference (RFI). The radio frequency interference that originates from a fast switching digital circuit will typically reach frequencies that are several decades above the lowest clock frequency of the other circuitry in the device. If the device contains a radio, there is a chance that part of the interference spectrum will land in the radio's operating band and will get picked up by its antenna subsystem; effectively desensitizing the radio.
One approach for suppressing self interference on the antenna system of the handheld device is to put a metal shield over all active circuits of the handheld device, the shield providing a physical and electrical barrier to the EMI; however, this can not be applied over the display and keyboard areas. Furthermore, if a circuit board (e.g., main logic board, or MLB) is longer than ¼ wavelength of the lowest radio frequency, the handheld device itself becomes a part of the antenna that may pick up EMI, and thus shielding is not effective unless the entire handheld device is covered by a shield.